1. Field of the Invention
This invention relates to photodetectors generally and more specifically to two-dimensional, integrated semiconductor image sensors.
2. Description of the Related Art
Small, integrated semiconductor image sensors are widely used to capture images and convert them to electronic signals, as for example in video cameras or electronic still-frame cameras. A variety of different digital image array formats are in current use, which provide a variety of different pixel densities. For example, proposed standards for High Density Television (HDTV) include pixel arrays of 1920 by 1080, 1280 by 720, or the lower resolution 640 by 360 (columns by rows).
For some applications, it is desirable to convert from one image format to another: for example, to convert a 1280 by 720 image into 640 by 360 format. Several methods are available to accomplish such a conversion. Conventional methods for converting formats include optical windowing, subsampling, and pixel aggregation by software manipulation. Each method has attendant disadvantages.
Optical windowing is perhaps the simplest and most obvious method of changing digital image formats. This method is simply using a smaller portion of the sensor array, for example the center portion, to capture the same image which was previously projected over the entire array. Although conceptually simple, this method is quite clumsy in practice. In order to shift and resize the image at the image plane, optical components must be moved and/or substituted to change the optical format. Such changes are difficult and expensive, and it is difficult to maintain adequate optical alignment. This solution is almost as difficult as simply substituting a completely new camera with the new format.
Subsampling of the image data is more convenient, as it does not require motion of physical, optical components. In this method, one converts from a higher density to a lower density format either during or after image acquisition, for example by software methods. After the image is digitized, for example, to change from 1280 columns to 640 columns, one can subsample by simply discarding every odd numbered column. One disadvantage to this method is that substantial information can be discarded, thereby compromising image quality. For example, if a highly periodic image were presented, in which every other column had a luminance of near maximum, that information might be discarded by subsampling. The resulting image would not accurately represent the original source image.
An alternative method, pixel aggregation, seeks to mitigate problems which accompany subsampling. Instead of subsampling, pixel aggregation averages adjacent pixels by software manipulation. One problem with this method is that only integer multiples of pixels can be aggregated. For example, one cannot easily convert 1920 rows to 1280 rows by aggregating, as the ratio 2/3 is not a whole integer ratio. Interpolation can be used, but some information is sacrificed by interpolation. Furthermore, computed interpolation is time consuming, particularly for large image arrays.
U.S. Pat. No. 5,262,871 teaches another alternative wherein the random addressing of pixels enables the readout of pixels located in selected regions of interest. In this method, relatively large groups of pixels are read out simultaneously and the resulting signals can be merged into superpixel signals. Once an area of interest is located, the number of pixels read during each cycle may be reduced to provide higher resolution, lower speed readout of the area of interest. Unfortunately, this method uses signal accumulation via charge aggregation on the signal bus. No means is provided for mitigating the attendant noise. The signal readout from each pixel is passive: i.e., no amplification is provided for either noise minimization or signal enhancement. Instead, the prior method uses digital control logic to selectively or collaterally address the pixels of interest.
In view of the above problems, the present invention provides a photodetector array with hardware-switchable resolution. The array includes a plurality of photodetectors, preferably photodiodes, coupled to a respective plurality of addressable interface circuits. At each pixel, a switching circuit configures neighboring ones of the photodetectors into pixels by summing multiple photodetector signals into an aggregated pixel output signal. The switching circuit is electronically switchable to aggregate said photodetector signals according to at least two different selectable pixellization schemes with differing resolution.
Preferably, control signals for the switching circuit are fabricated in polycrystalline silicon disposed underneath and in the shadows of metallization paths, for example addressing lines. Thus, no photoactive surface is consumed and fill factor is not diminished by the addition of the control signal paths.
In one particular embodiment, photodiodes are switchable into (1) pairs, or (2) groups of three neighboring photodiodes, in response to switching control signals. Thus, resolution is hardware switchable between (1) a maximum resolution, or (2) 2/3 of maximum resolution.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: